Field of the Disclosure
The present disclosure relates generally to wireless communication systems and, more particularly, to communication in unlicensed frequency bands of wireless communication systems.
Description of the Related Art
Unlicensed frequency bands are portions of the radiofrequency spectrum that do not require a license for use and may therefore be used by any device to transmit or receive radiofrequency signals. For example, the Unlicensed National Information Infrastructure (UNII) is formed of portions of the radio spectrum that include frequency bands in the range of 5.15 GHz to 5.825 GHz. For another example, the industrial, scientific, and medical (ISM) radio bands are portions of the radio spectrum that are reserved internationally for unlicensed communication. The ISM radio bands include bands with a center frequency of 2.4 GHz and a bandwidth of 100 MHz, a center frequency of 5.8 GHz and a bandwidth of 150 MHz, and a center frequency of 24.125 GHz and a bandwidth of 250 MHz, among other frequency bands. Unlicensed frequency bands can be contrasted to licensed frequency bands that are licensed to a particular service provider and may only be used for wireless communication that is authorized by the service provider or license holder.
Wireless communication devices that transmit or receive signals in licensed or unlicensed frequency bands are typically referred to as nodes, which may include Wi-Fi access points that operate according to IEEE 802.11 standards in the unlicensed spectrum. Nodes may also include base stations that operate in the licensed spectrum according to standards such as Long Term Evolution (LTE) standards defined by the Third Generation Partnership Project (3GPP). Base stations that operate according to LTE can implement supplementary downlink (SDL) channels in the unlicensed spectrum to provide additional bandwidth for downlink communications to user equipment that are also communicating with the base station using channels in a licensed frequency band. The licensed frequency bands may be referred to as LTE-L bands and the unlicensed frequency bands may be referred to as LTE-U bands. Base stations may also operate in the unlicensed frequency bands according to Licensed Assisted Access (LAA) standards. Base stations may also operate solely in the unlicensed frequency bands without support in licensed frequency bands, e.g., according to emerging standards such as MuLTEFire.
In dense networks, channels in the unlicensed frequency bands may be reused by nodes that operate according to different radio access technologies (RATs) such as Wi-Fi access points and LTE base stations. Communication by the nodes that operate according to the different RATs is coordinated to reduce interference between transmissions by the different nodes. For example, listen before talk (LBT) coexistence rules require that each node monitors a channel (e.g., “listens”) to detect energy on the channel prior to transmitting information on the channel. If the detected energy level is below a threshold level, the node is free to transmit on the channel for a predetermined time interval such as 4 milliseconds (ms) or 10 ms. If the detected energy level is above the threshold level, which indicates that another node is transmitting on the channel, the listening node is required to back off by a randomly determined time interval before making another attempt to acquire the channel. The energy detection threshold for Wi-Fi is −62 dBm, the energy detection threshold for LTE-U is −72 dBm, and the energy detection threshold for LAA and MuLTEFire is −72 dBm. Wi-Fi nodes may also perform Wi-Fi preamble decoding on signals with detected energy levels below the energy detection threshold with sensitivity of at least −82 dBm. The Wi-Fi node backs off if it successfully decodes preambles in transmissions by other Wi-Fi nodes.